Hubble Space Telescope Imaging of Luminous Extragalactic Infrared Transients and Variables from the Spitzer Infrared Intensive Transients Survey

Howard E. Bond, Jacob E. Jencson, Patricia A. Whitelock, Scott M. Adams, John Bally, Ann Marie Cody, Robert D. Gehrz, Mansi M. Kasliwal, Frank J. Masci

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The SPitzer InfraRed Intensive Transients Survey (SPIRITS) searched for luminous infrared (IR) transients and variables in nearly 200 nearby galaxies from 2014 to 2019, using the warm Spitzer telescope at 3.6 and 4.5 μm. Among the SPIRITS variables are IR-bright objects that are undetected in ground-based optical surveys. We classify them as (1) transients, (2) periodic variables, and (3) irregular variables. The transients include eSPecially Red Intermediate-luminosity Transient Events (SPRITEs), having maximum luminosities fainter than supernovae, red IR colors, and a wide range of outburst durations (days to years). Here we report deep optical and near-IR imaging with the Hubble Space Telescope (HST) of 21 SPIRITS variables. They were initially considered SPRITE transients, but many eventually proved instead to be periodic or irregular variables as more data were collected. HST images show most of these cool and dusty variables are associated with star-forming regions in late-type galaxies, implying an origin in massive stars. Two SPRITEs lacked optical progenitors in deep preoutburst HST images; however, one was detected during eruption at J and H, indicating a dusty object with an effective temperature of ∼1050 K. One faint SPRITE turned out to be a dusty classical nova. About half the HST targets proved to be periodic variables, with pulsation periods of 670-2160 days; they are likely dusty asymptotic-giant-branch (AGB) stars with masses of ∼5-10 M ⊙. A few of them were warm enough to be detected in deep HST frames, but most are too cool. Out of six irregular variables, two were red supergiants with optical counterparts in HST images; four were too enshrouded for HST detection.

Original languageEnglish (US)
Article number158
JournalAstrophysical Journal
Issue number2
StatePublished - Apr 1 2022

Bibliographical note

Funding Information:
R.D.G. was supported in part by the United States Air Force.

Funding Information:
P.A.W. acknowledges a research grant from the South African National Research Foundation and is grateful to John Menzies (SAAO) for the use of his period-finding software.

Funding Information:
Support for HST Program numbers GO-13935, GO-14258, and AR-15005 was provided by NASA through grants from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555.

Funding Information:
J.B. is supported by NSF grant AST-1910393.

Publisher Copyright:
© 2022. The Author(s). Published by the American Astronomical Society.


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